converters/libiconv:
- Remove const qualifier from iconv(3) to match POSIX:
http://pubs.opengroup.org/onlinepubs/9699919799/functions/iconv.html
- Patch iconv.h to expose more GNU extensions when LIBICONV_PLUG is
defined because the base system iconv supports these extensions too.
Add/remove patches to/from ports to call iconv with non-const arguments.
This breaks some ports on FreeBSD 10 because base system iconv.h still has
the const qualifier. Fix this by letting USES=iconv add a build dependency
on converters/libiconv so ports can use its iconv.h (with LIBICONV_PLUG
defined) instead of the base system iconv.h.
This exposed some ports that link with libiconv when it is available instead
of using libc iconv. In these cases one of the following changes has been

- Remove the DESTDIR modifications from individual ports as we have a new,
fully chrooted DESTDIR, which does not need such any more.
Sponsored by: Google Summer of Code 2007
Approved by: portmgr (pav)